1. Field of the Invention
The present invention relates to the field of process chemical baths, particularly hydrogen peroxide containing baths, and more particularly, to a scheme for monitoring and controlling an aqueous bath comprised of hydrogen peroxide and a second chemical component, which is either an acid or a base, wherein the bath is utilized in semiconductor manufacturing.
2. Prior Art
Chemical solutions have been utilized extensively for the manufacture of semiconductor devices. Wet chemical processing baths have been used for cleaning semiconductor wafers, as well as for etching deposited films on these wafers. For example, the use of hydrogen peroxide (H.sub.2 O.sub.2) containing solutions for cleaning silicon semiconductor wafers is well known. In addition to wafer cleaning, hydrogen peroxide is utilized in combination with sulfuric acid for photoresist removal and in combination with phosphoric acid, sulfuric acid or ammonium hydroxide for selective titanium etching.
It is known that to ensure uniform processing in advanced VLSI (very large scale integrated circuit) and ULSI (ultra large scale integrated circuit) manufacturing, it is critical to maintain a chemical composition of a bath at a specified concentration level. Alternatively, more uniform processing can be attained by measuring the solution concentration and adjusting the wafer processing time to compensate for changes in the solution composition. Maintaining specified concentration levels is especially complicated in hydrogen peroxide based solutions.
For example, a NH.sub.4 OH--H.sub.2 O.sub.2 --H.sub.2 O solution (commonly referred to as SC-1) used in wafer cleaning, especially in megasonic baths, provides for the simultaneous removal of particles, organics and a number of trace metals (See for example, "Cleaning Solutions Based on Hydrogen Peroxide for use in Silicon Semiconductor Technology"; W. Kern and D. A. Puotinen; RCA Review, June 1970; pp. 187-206). In the SC-1 solution the bath is comprised of ammonium hydroxide (NH.sub.4 OH), hydrogen peroxide (H.sub.2 O.sub.2) and water (H.sub.2 O). For semiconductor manufacturing, it is imperative to maintain the proper chemistry ratio of NH.sub.4 OH--H.sub.2 O.sub.2 --H.sub.2 O in the bath. If the NH.sub.4 OH/H.sub.2 O.sub.2 ratio is high, silicon etching can occur, resulting in the generation of surface roughness that can adversely affect gate oxide breakdown. If the NH.sub.4 OH/H.sub.2 O.sub.2 ratio is too low, the particle removal rate is reduced and a higher iron contamination level may result.
Furthermore, many solutions, such as the SC-1 solution, are notoriously unstable due to the simultaneous loss of multiple components of the bath. In the SC-1 processing bath, NH.sub.3 and H.sub.2 O.sub.2 losses are attributable to a number of factors. The H.sub.2 O.sub.2 decomposition (2H.sub.2 O.sub.2 &gt;2H.sub.2 O+O.sub.2) is dependent on concentration, pH and temperature. It is also well known that the presence of heavy metal contaminants affect the decomposition of H.sub.2 O.sub.2. It is suspected that organic contaminants can affect the ratio of decomposition. For ammonia, the loss is primarily due to evaporation. It is also believed that some ammonia loss may be due to oxidation (2NH.sub.3 +6OH.sup.- &gt;N.sub.2 +6H.sub.2 O+6e.sup.-).
Thus, at most semiconductor fabrication facilities, a common approach has been to use a liquid processing bath for a certain time period without proper concentration adjustments to make up for the losses and then to discard it. This practice not only results in high chemical costs, but it also leads to the generation of more waste than would be required. Environmentally, it is preferred to reduce such waste. In more advanced manufacturing facilities, automated controllers are utilized to achieve some degree of chemical composition control. These controllers spike the bath with certain chemicals at predefined intervals and can also add one or more chemicals to the bath to make up for a drop in the bath liquid level. One automated approach is described in U.S. Pat. No. 4,326,940.
In the manufacture of state-of-the-art and future generations of semiconductor devices, it is appreciated that the specified tolerances for chemical composition in such baths will require tighter tolerances. In order to manufacture even smaller submicron semiconductor devices, as well as improving the manufacturing yield, it is imperative that automated schemes for maintaining a tighter control on the chemical make-up of a liquid processing bath is desired. In order to ensure uniform processing, such as uniform cleaning without surface damage, stripping and/or etching, it is imperative to continually monitor and, if necessary, appropriately adjust the concentration level of chemicals in an aqueous bath without human interaction.
In respect to SC-1 baths, a more advanced concentration control for H.sub.2 O.sub.2 and NH.sub.3 is desirable. What is needed is a workable sensor system for use in real-time process control. Characteristics which are desirable in both H.sub.2 O.sub.2 and NH.sub.3 sensors include simplicity, sensitivity, accuracy, reproducibility, dynamic range, speed, robustness, low cost, and commercial availability. Such sensors must be capable of withstanding the effects of a corrosive liquid at elevated temperatures and provisions are necessary to deal with the high concentration of bubbles which are present in this type of solution.
The present invention provides for a scheme in which substantially continuous, frequently repetitive and/or in-situ monitoring of a chemical bath, such as the SC-1 bath or a SC-2 bath (an SC-2 solution is comprised of HCl--H.sub.2 O.sub.2 --H.sub.2 O), is obtained to determine the concentration levels of the chemical species. Furthermore, such measured data can be analyzed and computed in order to automatically control the composition of the bath. Moreover, this scheme provides for an analytical methodology which can be most easily implemented in a semiconductor fabrication facility with reasonable cost, desired analytical accuracy and a lower probability of introducing deleterious contamination within the facility.
Additionally, it is appreciated that the concentration monitoring scheme described in the afore-mentioned original application can be utilized to provide the necessary control. However, it is also appreciated that added complexities are encountered in generating a diluted sample for analysis. For example, errors in the measurement of the diluted sample are multiplied when interpolated to calculate the particular chemical concentration in the bath. Additionally, the use of reagents and carrier streams are complicated by the requirement of additional plumbing, as well as the requirement of precision controls in manipulating the fluid flow. Thus, in some instances the dilution scheme of the original application operates effectively to provide the desired results, but in other instances it is preferable to obtain direct readings from a more simplified scheme. The present invention attempts to circumvent many of these complexities by utilizing a direct measurement scheme.